To date, for piping paths, of semiconductor manufacturing apparatuses, liquid crystal manufacturing apparatuses, and the like, which are required to have chemical resistance or the like, tubes or pipe joints made of synthetic resin such as fluororesin have been used. As such a pipe joint, a pipe joint that includes: a joint body to be mounted to a leading end portion of a tube; and a union nut having a female screw portion to be tightened onto a male screw portion formed on the outer circumferential portion of the joint body, is often used. The pipe joint has a structure in which the union nut is tightened onto the outer circumferential portion of the joint body, to assuredly obtain a sealing performance between the joint body and the tube.
For example, in a pipe joint made of resin as described in Patent Literature 1, an inner ring (fitting portion) is pressed into a leading end portion of a tube (tubing) made of synthetic resin such that a fluid path is formed by an inner circumferential portion of the inner ring, and the inner ring has a protrusion that protrudes radially outward. The tube, which has been deformed so as to have its diameter enlarged by the protrusion of the inner ring being pressed into the tube, is inserted into a receiver opening of a joint body. In this state, a female screw portion formed in the inner circumference of a union nut (pressing ring) is screwed onto a male screw portion formed on the outer circumference of the joint body, whereby the tube and the inner ring are pressed in the axial direction by the union nut, and the tube is connected to the joint body.
In the above structure, in order to assuredly obtain a favorable sealing performance, a degree of tightening of the union nut onto the joint body needs to be controlled so as to represent an appropriate value. As a method for controlling a degree of tightening, for example, a method for controlling a tightening torque by gradually tightening the union nut by using a tool such as a spanner, or a method for controlling a degree of tightening while repeatedly measuring a tightening extent with the use of a feeler gauge or the like, is known.